This invention relates generally to apparatus for converting images recorded in two-dimensional media as variations in either reflectivity or transmissivity into digital data which may be processed by a computer, communicated to another location, or reproduced as visual images that are copies of the original images. More specifically, the invention relates to image digitizing apparatus that can accurately digitize images having extremely wide dynamic ranges.
The apparatus to which the present invention is addressed is of the type where the picture elements ("pixels") constituting the image are imaged either in time sequence on a single light detector or in groups in time sequence on a group of detectors, each pixel in a group being imaged on a particular detector in the detector group on a one-to-one basis. Thus, the apparatus of interest includes pixel-by-pixel scanners, line-by-line scanners, and apparatus in which there is a dedicated detector for each pixel in the image.
The outputs of the light detectors are digitized and translated into reflectivity or transmissivity values after which they are read out to an associated computer for further processing, storage, reproduction as visual copies of the original, or transmission to another location.
Image digitizing apparatus is comprised of a visible light source that illuminates the image to be digitized or a portion thereof, one or more visible light detectors, a lens system that images all or a portion of the pixels that constitute the image on associated light detectors, a transport mechanism for positioning the image containing medium relative to detectors, light source, and lens system (or vice versa) in those situations where portions of the image are read out in time sequence, a time-division multiplexer for assembling the detector data into a serial stream, analog-to-digital circuitry for converting the detector outputs into digital formats, and a digital processor for converting the pixel data into reflectivity or transmissivity values and controlling all of the elements of the digitizing apparatus so that they work properly together.
Inherent device limitations limit the dynamic range of solid-state visible light detectors to less than a few hundred to one while certain types of images involve dynamic ranges that are higher by an order of magnitude or more. Thus, there is a need for image digitizing apparatus that can utilize the low-cost, low-dynamic range solid-state detectors that are currently available and yet realize the high dynamic ranges that are necessary for the precise digital conversion of certain types of imagery.